In a nutshell you have to add them yourself, one way you can do this is at your addon airports you will have flightplans not compiling due not enough gates, you can open TG click the departure tab at the top scroll down to the CODE for your airport then edit a plan, example change the aircraft from EasyJet 737 to ABY FDX FedEx A300-600 or any cargo you like, recompile then check, I have added over 1500 Cargo aircraft using this method.